In recent years, attention has been focused on MIMO (Multiple-Input Multiple-Output) as an art for realizing high-speed large-capacity communications in a radio communication technology. The MIMO is an art of transmitting data using a plurality of antennas in both transmission and reception. Different data are transmitted from a plurality of transmission antennas respectively, whereby the transmission capacity can be enhanced without enlarging the time and frequency resources.
In the MIMO, when data are transmitted from a plurality of antennas, a beam transmission method of forming a beam by transmitting weighted data from each antenna is available. The beam transmission has the advantage that the reception power of a terminal is increased due to the beam gain.
Spatial multiplexing using a plurality of beams is also possible, in which case beam transmission suited for the state of a propagation channel is performed, whereby the transmission capacity can be improved as compared with spatial multiplexing using antennas. In this case, it is necessary to send information of a beam suited for the propagation channel state of the receiving party to the transmission side.
At present, in 3GPP (3rd Generation Partnership Project) of an international standardization organization of mobile telephones, standardization activity of an LTE (Long Term Evolution) system is carried out as a system for realizing higher-speed larger-capacity communications than communications of the current third generation mobile telephone. Also in the LTE, the MIMO is positioned as an indispensable technology to meet the required condition for high-speed large-capacity transmission. In the LTE, the transmission beam technology is described as a technology of pre-coding.
FIG. 11 schematically shows transmission beam formation by pre-coding. For example, to use it for a cellular system for mobile communications of mobile telephones, etc., when a signal is transmitted through multiple antennas from a radio base station 201, the beam number indicating the optimum beam pattern in a user equipment 202 is set according to a feedback signal from the user equipment 202 of a mobile station. The radio base station 201 weights the transmission signal of each antenna in response to the beam number, thereby forming a transmission beam which becomes a beam pattern in an appropriate direction for the receiving user equipment 202. In the example in FIG. 11, four beams of beams ‘a’ to ‘d’ can be formed as beam numbers; to select two beams from among them for use, for example, two transmission beams of beams ‘b’ and ‘c’ are formed according to a feedback signal from the user equipment 202.
According to the pre-coding, a beam responsive to the feedback signal from the user equipment is selected for use among the stipulated beam patterns transmitted from the radio base station, whereby the signal strength when a signal is transmitted to the user equipment can be ensured and in addition, it is made possible to transmit a plurality of signals at the same time using orthogonal beams. Thus, the advantages of coverage enlargement due to the improvement in the signal strength and improvement in throughput due to the space division can be expected.
In the pre-coding, to make a selection of a plurality of beams (which will be hereinafter represented as “subset selection”), the information amount to represent all combinations that can be taken becomes large and thus a large signaling amount, namely, many resources become necessary as control information. Specifically, to select R beams from N beams, NCR combinations become necessary; for example, to select three beams from six beams, five bits are required to represent 6C3=20 candidates.    Non-patent document 1: 3GPP TSG RAN WG1 #42, R1-050889, Samsung, “MIMO for Long Term Evolution”, Aug. 29-Sep. 2, 2005